The long, slow decay of carbon-14 allows archaeologists to accurately date
the relics of history back to 60,000 years.
And while the carbon dating technique is well known and understood, the
reason for carbon-14’s slow decay has not been understood. Why, exactly, does
carbon-14 have a half-life of nearly 6,000 years while other light atomic
nuclei have half-lives of minutes or seconds?
“This has been a very significant puzzle to nuclear physicists for
several decades,” said James Vary, an Iowa State Univ. professor of physics
and astronomy. “And the underlying reason turned out to be a fairly exotic
one.”
The reason involves the strong three-nucleon forces within each carbon-14
nucleus. It’s all about the simultaneous interactions among any three nucleons
and the resulting influence on the decay of carbon-14. And it’s no easy task to
simulate those interactions.
In this case, it took about 30 million processor-hours on the Jaguar
supercomputer at Oak Ridge National Laboratory in Tennessee.
The research project’s findings were
recently published online by Physical
Review Letters.
Vary and Pieter Maris, an Iowa
State research staff
scientist in physics and astronomy, are the lead authors of the paper.
Collaborating on the paper are Petr Navratil of TRIUMF (Canada’s National
Laboratory for Particle and Nuclear Physics in Vancouver) and the Lawrence
Livermore National Laboratory in California; Erich Ormand of Lawrence Livermore
National Lab; plus Hai Ah Nam and David Dean of Oak Ridge National Lab. The
research was supported by contracts and grants from the U.S. Department of
Energy Office of Science.
Vary, in explaining the findings, likes to remind people that two subatomic
particles with different charges will attract each other. Particles with the
same charges repel each other. Well, what happens when there are three
particles interacting that’s different from the simple addition of their
interactions as pairs?
The strong three-nucleon interactions are complicated, but it turns out a
lot happens to extend the decay of carbon 14 atoms.
“The whole story doesn’t come together until you include the
three-particle forces,” said Vary. “The elusive three-nucleon forces
contribute in a major way to this fact of life that carbon-14 lives so
long.”
Maris said the three-particle forces work together to cancel the effects of
the pairwise forces governing the decay of carbon-14. As a result, the
carbon-14 half-life is extended by many orders of magnitude. And that’s why
carbon-14 is a very useful tool for determining the age of objects.
To get that answer, Maris said researchers needed a billion-by-billion
matrix and a computer capable of handling its 30 trillion non-zero elements.
They also needed to develop a computer code capable of simulating the entire
carbon-14 nucleus, including the roles of the three-nucleon forces.
Furthermore, they needed to perform the corresponding simulations for
nitrogen-14, the daughter nucleus of the carbon-14 decay. And, they needed to
figure out how the computer code could be scaled up for use on the Jaguar
petascale supercomputer.
“It was six months of work pressed into three months of time,”
Maris said.
But it was enough for the nuclear physicists to explain the long half-life
of carbon-14. And now they say there are more puzzles to solve:
“Everybody now knows about
these three-nucleon forces,” Vary said. “But what about four-nucleon
forces? This does open the door for more study.”
